1. Field of the Invention
This invention relates to circuit breakers having a magnetic trip assembly in which the magnetic field induced by an abnormal current unlatches a latchable operating mechanism to trip the breaker, and more particularly to such a magnetic trip assembly which is responsive to low levels of overcurrent.
2. Background Information
A common type of circuit breaker used to automatically interrupt abnormal currents in an electrical system incorporates a thermal trip device which responds to persistent low levels of overcurrent and a magnetic trip assembly which responds instantly to higher levels of overcurrent. An example of such a circuit breaker is disclosed in U.S. Pat. No. 3,849,747. In such circuit breakers, the thermal trip device comprises a bimetal which bends in response to the persistent low level overcurrent passed through it to unlatch a latchable operating mechanism. The latchable operating mechanism is spring operated to open electrical contacts which interrupt the current. The magnetic trip assembly includes an armature which is spring biased to latch the operating mechanism. The current through the bimetal produces a magnetic field which is concentrated by a magnetic yoke to attract the armature and unlatch the operating mechanism at a specified level of overcurrent. The bimetal in these circuit breakers acts as a one turn electromagnet for the magnetic trip assembly.
Such circuit breakers have been in use for many years and their design has been refined to provide an effective, reliable circuit breaker which can be easily and economically manufactured on a large scale.
Recently there has developed a market for such circuit breakers with a magnetic trip assembly which operates at lower levels of instantaneous overcurrent. The level of overcurrent at which the magnetic trip operates is a function of several factors, including the friction force applied by the spring operated latchable operating mechanism to the armature of the magnetic trip assembly, the spring constant of the spring biasing the armature to latch the operating mechanism, the magnitude of the magnetic field produce by the overcurrent and the coupling of the magnetic field to the armature. One approach to lowering the level of overcurrent at which the magnetic trip operates is to loop a wire providing current to the bimetal around the magnetic yoke to increase the ampere turns of the electromagnet, and therefore, the strength of the field. However, such an approach adds complexity to the circuit breaker mechanism thereby adding steps and cost to the manufacturing process.
There remains a need, therefore, for an improved circuit breaker with a low magnetic trip.
There is a further need for such a circuit breaker which can be produced economically.
There is a related need for a circuit breaker with a low magnetic trip which requires little modification to the existing circuit breaker designs.